Lithium secondary batteries (or cells) have been widely accepted for use in portable power applications. Portable power applications refer to uses covering such application as laptop computers, cellular phones, and other portable devices. These devices require relatively small batteries and one industry standard is referred to as an ‘18650’ cell, which has the nominal dimensions of 65 mm×18 mm diameter. Additionally, lithium secondary batteries (also referred to as lithium rechargeable batteries or secondary lithium ion batteries) are also particularly well suited for larger applications, such as, for example, electric vehicle applications, because of their high-energy capacity and their relatively low weight.
Lithium secondary batteries for electric vehicle applications or other applications requiring greater amounts of energy, however, may have to be significantly larger in size (i.e. volumetric). This is because the battery will, for example, be used as the energy source for the electric motor that will propel the vehicle. Since the energy required to propel the vehicle is significantly greater than that needed to energize the portable devices mentioned above, the volumetric size of the vehicle battery will be larger.
One issue surrounding lithium cells is that the lithium is a highly reactive metal, which is capable of igniting at a temperature of about 160° C. These batteries are typically made by packing the components, including the lithium-laden components, into a sealed metal can. The sealed can may rupture as a result of short-circuiting (i.e., direct contact) between the anode and cathode material. To prevent short-circuiting or minimize its consequences, a battery separator is placed between the anode and cathode to prevent direct contact.
In the commercially available lithium cells (e.g., the 18650 cell), heat generation during operation (as a result of charging and discharging) is not a significant problem as the small size lends itself well to dissipation of the heat. However, as the volume of the cell grows, dissipation of heat from the cell becomes a more significant problem. In the commercially available cells (e.g., 18650), the separators are typically made of microporous polyolefin films, which have a tendency to shrink. This is, in part, a result of the stretching required to make the microporous film.
Accordingly, there is a need to have a more dimensionally stable (or high temperature melt integrity) separator for larger cells, because if short-circuiting occurs, the rupture of the cell could be more significant because of the greater mass of lithium material contained in the larger cell.
In the art, it is known to combine microporus films and nonwoven materials to form a battery separator. See U.S. Pat. No. 6,511,774, ‘Background Art,’ and commercially available products, such as CELGARDO® 4000 and 5000 Series products from Celgard, Inc., Charlotte, N.C. These products are made by laminating (i.e., application of heat and pressure by smooth or patterned nip rollers) the nonwoven directly to the film. However, these products can only provide dimensional stability up to 167° C. (the melting point of the polypropylene nonwoven and membrane components). In addition, their ion-transport channels are blocked during lamination and thereby reduce the efficiency of the battery.
Accordingly, there is a need to find a way to bond together the nonwoven with the high melt integrity and mircoporous film that will not inhibit ion flow between the anode and cathode and will maintain its dimensional stability at a temperature above 167° C.